I. Field of the Invention
The present invention relates to wireless communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for processing a pilot channel that has sign ambiguities resulting from puncturing.
II. Description of the Related Art
In a wireless radiotelephone communication system, many users communicate over a wireless channel. Communication over the wireless channel can be one of a variety of multiple access techniques that allow a large number of users in a limited frequency spectrum. These multiple access techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
The CDMA technique has many advantages. An exemplary CDMA system is described in U.S. Pat. No. 4,901,307, entitled "Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters", issued Feb. 13, 1990, assigned to the assignee of the present invention, and incorporated herein by reference. An exemplary CDMA system is further described in U.S. Pat. No. 5,103,459, entitled "System And Method For Generating Signal Waveforms In A CDMA Cellular Telephone System", issued Apr. 7, 1992, assigned to the assignee of the present invention, and incorporated herein by reference.
In each of the above patents, the use of a forward-link (base station to mobile station) pilot signal is disclosed. In a typical CDMA wireless communication system, such as that described in EIA/TIA IS-95, the pilot signal is a "beacon" transmitting a constant data value and spread with the same pseudonoise (PN) sequences used by the traffic bearing signals. The pilot signal is typically covered with the all-zero Walsh sequence. During initial system acquisition, the mobile station searches through PN offsets to locate a base station's pilot signal. Once it has acquired the pilot signal, it can then derive a stable phase and magnitude reference for coherent demodulation, such as that described in U.S. Pat. No. 5,764,687 entitled "Mobile Demodulator Architecture For A Spread Spectrum Multiple Access Communication System," issued Jun. 9, 1998, assigned to the assignee of the present invention, and incorporated herein by reference.
Recently, third-generation (3G) wireless radiotelephone communication systems have been proposed in which a reverse-link (mobile station to base station) pilot channel is used. For example, in the currently proposed cdma2000 standard, the mobile station transmits a Reverse Link Pilot Channel (R-PICH) that the base station uses for initial acquisition, time tracking, rake-receiver coherent reference recovery, and power control measurements. Such a reverse link pilot is described in detail in co-pending U.S. patent application Ser. No. 08/886,604, entitled "HIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEM", assigned to the assignee of the present invention and incorporated by reference herein.
Punctured into the R-PICH is a Forward Link Power Control Subchannel that carries information on the quality of the forward link (base station to mobile station) as received at the mobile station. This information is used by the base station to adjust the power of the forward link channels that particular mobile station is receiving. The generation of power control measurements by a base station is shown in U.S. Pat. No. 5,056,109, entitled "Method And Apparatus For Controlling Transmission Power In A CDMA Cellular Mobile Telephone System", issued Oct. 8, 1991, assigned to the assignee of the present invention and incorporated herein by reference.
FIG. 1 illustrates an exemplary circuit for generation of the R-PICH in a cdma2000 system. Pilot signal generator 102 generates a constant data value of +1. Power control bit generator 104 generates either a +1 or a -1 data value depending whether the mobile station is experiencing an acceptable frame error rate. In the cdma2000 system, power control bit generator 104 generates one power control bit every 1.25 ms, which equates to one power control bit per power control group. Symbol repeater 106 generates multiple chips representative of the power control bit. In the cdma2000 system, the number of chips generated by the symbol repeater 106 is an integer multiple of 384, depending on the spreading rate. Multiplexer 108 constructs the R-PICH by multiplexing the +1 data value generated by pilot signal generator 102 with the repeated power control chips output from symbol repeater 106. Specifically, multiplexer 108 constructs a stream of power control groups according to FIG. 2.
FIG. 2 illustrates a single power control group 202 of the R-PICH of a cdma2000 system. The sub-blocks 202A-202C each correspond to an integer multiple of 384 chips of the +1 data value generated by pilot signal generator 102 (FIG. 1). The sub-block 202D corresponds to an integer multiple of 384 chips of either the +1 or -1 power control chips generated by power control bit generator 104 and repeated by symbol repeater 106. Thus, as can be seen from FIGS. 1 and 2, in the cdma2000 system, the R-PICH comprises a sequence of +1 data values, multiplexed with forward link power control information of unknown sign. In other words, although the first three-quarters of each power control group 202 are of known sign (+1), the last quarter is of unknown sign. A similar situation (i.e., sign or phase uncertainty) occurs in other wireless communication systems that use a discontinuous or punctured pilot channel.
Because the R-PICH is used by the base station as a coherent reference for data demodulation, a frequency reference for frequency tracking, and a received power reference for power control measurements, the uncertainty introduced in the otherwise constant data value by the forward link power control subchannel introduces possible degradations in performance. In other words, the puncturing in of the forward link power control subchannel into the R-PICH degrades the reverse link performance (as measured by the base station) with respect to a continuous pilot channel, For example, one major effect of the punctured pilot channel is that signal to noise ratio of the channel phase estimate calculated by the base station is decreased, resulting in an average loss of 0.3 dB in required E.sub.b /N.sub.0 for a given frame error rate (FER) of 1% if the sign uncertainty of the forward link power control subchannel is not used in the channel phase estimate (i.e., the corresponding power control chips are blanked before being processed by the channel phase estimator).
Thus, a method and apparatus is needed for preventing these degradations in reverse link performance caused by puncturing information bits of uncertain sign into an otherwise continuous pilot channel.